Well folks - I have checked with dealers and am informed that the rear airbags are definitely an "extra". The remarkable thing is that the panel covers have the "Airbag" badge on them. BMW-AG advise that they should not be there there should just be a plain insert not one marked with "Airbag".
I have found a few other things that re not quite correct with the car though - such as no heat to drivers footwell because the ducting is not properly fitted and has a broken latch slot so will never be able to connect correctly. Will demand it be replaced when next in to dealer with the rest of the list...!
Thanks for the comments, I suppose I was right to be confused when a panel is marked "Airbag" when it shouldn't have been - on TWO new cars...!
Thanks again.